Encoding apparatus for signaling an extension directional intra-prediction mode within a set of directional intra-prediction modes

ABSTRACT

An encoding apparatus is described for signaling an extension directional intra-prediction mode within a set of directional intra-prediction modes, the set of directional intra-prediction modes comprising predetermined directional intra-prediction modes and the extension directional intra-prediction mode. The encoding apparatus comprises a mode mapping unit selecting a predetermined directional intra-prediction mode, the selected predetermined directional intra-prediction mode being associated with an intra mode index, and mapping the extension directional intra-prediction mode onto the selected predetermined directional intra-prediction mode. A signaling unit generates a signaling indicator comprising at least one of the intra mode index and a flag value. An intra-prediction unit intra-predicts pixel values of pixels of a rectangular video coding block on the basis of the extension directional intra-prediction mode for providing a predicted rectangular video coding block. An encoding unit encodes the rectangular video coding block on the basis of the predicted rectangular video coding block.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/RU2016/000918, filed on Dec. 23, 2016, the disclosure of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

Generally, the present disclosure relates to the field of video coding.More specifically, the present disclosure relates to an encodingapparatus for signaling an extension directional intra-prediction modefor directional intra-prediction of a video coding block using asignaling indicator and a decoding apparatus for handling the signalingindicator.

BACKGROUND

Digital video communication and storage applications are implemented bya wide range of digital devices, e.g. digital cameras, cellular radiotelephones, laptops, broadcasting systems, video teleconferencingsystems, etc. One of the most important and challenging tasks of theseapplications is video compression. The task of video compression iscomplex and is constrained by two contradicting parameters: compressionefficiency and computational complexity. Video coding standards, such asITU-T H.264/AVC or ITU-T H.265/HEVC, provide a good tradeoff betweenthese parameters. For that reason support of video coding standards is amandatory requirement for almost any video compression application.

The state-of-the-art video coding standards are based on partitioning ofa source picture into video coding blocks (or short blocks). Processingof these blocks depend on their size, spatial position and a coding modespecified by an encoder. Coding modes can be classified into two groupsaccording to the type of prediction: intra- and inter-prediction modes.Intra-prediction modes use pixels of the same picture (also referred toas frame or image) to generate reference samples to calculate theprediction values for the pixels of the block being reconstructed.Intra-prediction is also referred to as spatial prediction.Inter-prediction modes are designed for temporal prediction and usesreference samples of previous or next pictures to predict pixels of theblock of the current picture. After a prediction stage, transform codingis performed for a prediction error that is the difference between anoriginal signal and its prediction. Then, the transform coefficients andside information are encoded using an entropy coder (e.g., CABAC forAVC/H.264 and HEVC/H.265). The recently adopted ITU-T H.265/HEVCstandard (ISO/IEC 23008-2:2013, “Information technology—High efficiencycoding and media delivery in heterogeneous environments—Part 2: Highefficiency video coding”, November 2013) declares a set ofstate-of-the-art video coding tools that provide a reasonable tradeoffbetween coding efficiency and computational complexity. An overview onthe ITU-T H.265/HEVC standard has been given by Gary J. Sullivan,“Overview of the High Efficiency Video Coding (HEVC) Standard”, in IEEETransactions on Circuits and Systems for Video Technology, Vol. 22, No.12, December 2012, the entire content of which is incorporated herein byreference.

Similarly to the ITU-T H.264/AVC video coding standard, the HEVC/H.265video coding standard provides for a division of the source picture intoblocks, e.g., coding units (CUs). Each of the CUs can be further splitinto either smaller CUs or prediction units (PUs). A PU can be intra- orinter-predicted according to the type of processing applied for thepixels of PU. In case of inter-prediction, a PU represents an area ofpixels that is processed by motion compensation using a motion vectorspecified for a PU. For intra prediction, the adjacent pixels ofneighbor blocks are used as reference samples to predict a currentblock. A PU specifies a prediction mode that is selected from the set ofintra-prediction modes for all the transform units (TUs) contained inthis PU. A TU can have different sizes (e.g., 4×4, 8×8, 16×16 and 32×32pixels) and can be processed in different ways. For a TU, transformcoding is performed, i.e. the prediction error is transformed with adiscrete cosine transform or a discrete sine transform (in theHEVC/H.265 standard, it is applied to intra-coded blocks) and quantized.Hence, reconstructed pixels contain quantization noise (it can becomeapparent, for examples, as blockiness between units, ringing artifactsalong with sharp edges, etc.) that in-loop filters such as DeblockingFilter (DBF), Sample Adaptive Offset (SAO) and Adaptive Loop Filter(ALF) try to suppress. The use of sophisticated prediction coding (suchas motion compensation and intra-prediction) and partitioning techniques(e.g., quadtree for CUs and PUs as well as residual quadtree for TUs inthe HEVC/H.265 standard and quadtree plus binary tree for the JEMreference software starting from version JEM-3.0) allowed thestandardization committee to significantly reduce the redundancy in PUs.

According to the HEVC/H.265 standard, the intra prediction modes asshown in FIG. 5 include a planar mode (the intra-prediction mode indexis 0), DC mode (the intra-prediction mode index is 1), and 33directional modes (the intra-prediction mode index ranges from 2 to 34,indicated by the solid lines). The set of directional intra-predictionmodes was extended up to 65 modes (almost doubled) by decreasing a stepangle between directional intra-prediction modes by a factor of 2. Thedotted lines in FIG. 5 denote the angular modes, which are introduced inthe JEM software.

For the JEM-3.0 software, a new partitioning mechanism based on bothquad-tree and binary-tree (known as QTBT) was proposed. The fundamentaldifference between the QT and QTBT partitioning mechanisms is that thelatter one enables not only square but also rectangular blocks by usingpartitioning based on both quad- and binary-tree. FIG. 6 illustrates anexample of block partitioning and a corresponding tree structure byusing QTBT, wherein solid lines denote quad-tree partitioning and dashedlines denote binary-tree partitioning. In each partitioning node of thebinary-tree, the partitioning type is indicated by 0 (horizontalpartitioning) or 1 (vertical partitioning).

Some signaling overhead and increased computational complexity at theencoder side are the price of the QTBT partitioning as compared toconventional quad-tree based partitioning used in the HEVC/H.265standard. Nevertheless, the QTBT-based partitioning is endowed withbetter segmentation properties and demonstrates significantly highercoding efficiency than the conventional quad-tree (“EE2.1: Quadtree plusbinary tree structure integration with JEM tools,” ContributionJVET-00024 to the 3^(rd) JVET meeting, Geneva, Switzerland, May 2016 byHan Huang, Kai Zhang, Yu-Wen Huang, Shawmin Lei). However, the QTBTpartitioning has a critical problem: a set of available directionalintra-prediction modes has not been changed accordingly. Thus, theasymmetry nature of rectangular blocks utilized by the QTBT frameworkhas not been taken into account, as shown in FIG. 7, i.e., the samenumber of reference samples are used along both shorter and longer sidesof rectangular blocks. Therefore, the number of directionalintra-prediction modes depends on neither aspect ratio of blocks noractual availability of reference samples in the current implementationof the QTBT framework.

In light of the above, there is a need for apparatuses and methods forvideo coding, which allow for an efficient handling of rectangular videocoding blocks.

SUMMARY

It is an object to provide apparatuses and methods for video coding,which allow for an efficient handling of rectangular video coding blocksin conjunction with a directional intra-prediction mechanism.

The foregoing and other objects are achieved by the subject matter ofthe independent claims. Further implementation forms are apparent fromthe dependent claims, the description and the figures.

The following disclosure employs a plurality of terms which, inembodiments, have the following meaning: Slice—a spatially distinctregion of a picture that is independently encoded/decoded. Sliceheader—Data structure configured to signal information associated with aparticular slice. Video coding block (or short block)—an M×N (M-columnby N-row) array of pixels or samples (each pixel/sample being associatedwith at least one pixel/sample value), or an M×N array of transformcoefficients. Coding Tree Unit (CTU) grid—a grid structure employed topartition blocks of pixels into macro-blocks for video encoding. CodingUnit (CU)—a coding block of luma samples, two corresponding codingblocks of chroma samples of an image that has three sample arrays, or acoding block of samples of a monochrome picture or a picture that iscoded using three separate color planes and syntax used to code thesamples. Picture Parameter Set (PPS)—a syntax structure containingsyntax elements that apply to zero or more entire coded pictures asdetermined by a syntax element found in each slice segment header.Sequence Parameter Set (SPS)—a syntax structure containing syntaxelements that apply to zero or more entire coded video sequences asdetermined by the content of a syntax element found in the PPS referredto by a syntax element found in each slice segment header. VideoParameter Set (VPS)—a syntax structure containing syntax elements thatapply to zero or more entire coded video sequences. Prediction Unit(PU)—a prediction block of luma samples, two corresponding predictionblocks of chroma samples of a picture that has three sample arrays, or aprediction block of samples of a monochrome picture or a picture that iscoded using three separate color planes and syntax used to predict theprediction block samples. Transform Unit (TU)—a transform block of lumasamples, two corresponding transform blocks of chroma samples of apicture that has three sample arrays, or a transform block of samples ofa monochrome picture or a picture that is coded using three separatecolor planes and syntax used to predict the transform block samples.Supplemental enhancement information (SEI)—extra information that may beinserted into a video bit-stream to enhance the use of the video.Luma—information indicating the brightness of an image sample.Chroma—information indicating the color of an image sample, which may bedescribed in terms of red difference chroma component (Cr) and bluedifference chroma component (Cb).

Generally, the present disclosure relates to an apparatus and a methodfor improving the directional intra-prediction mechanism within the QTBTframework. More specifically, the present disclosure extends a set ofavailable directional intra-prediction modes subject to the aspect ratioof a block to be predicted, enables or disables some directionalintra-prediction modes subject to the availability of reference samples,signals directional intra-prediction modes contained in the extendedsubset via mode mapping and a one-bit flag.

Embodiments of the present invention provide, amongst others, thefollowing advantages: additional coding gain after integrating thistechnique into a codec, extensive applications in hybrid video codingparadigms compatible with the HM software and the VPX video codec familyas well as in the state-of-the-art and next-generation video codingframeworks (the JEM software and VPX/AV1 video codec familyrespectively), low hardware and computational complexities at bothencoder and decoder sides, easy implementation in such codecs that useconventional directional intra-prediction mechanisms.

According to a first aspect, the invention relates to an encodingapparatus for encoding a rectangular video coding block, the encodingapparatus being configured to signal an extension directionalintra-prediction mode within a set of directional intra-prediction modesusing a signaling indicator, the set of directional intra-predictionmodes comprising predetermined directional intra-prediction modes andthe extension directional intra-prediction mode. The encoding apparatuscomprises a mode mapping unit configured to select a predetermineddirectional intra-prediction mode from the set of directionalintra-prediction modes in dependence of the extension directionalintra-prediction mode, the selected predetermined directionalintra-prediction mode being associated with an intra mode index, and tomap the extension directional intra-prediction mode onto the selectedpredetermined directional intra-prediction mode, a signaling unitconfigured to generate the signaling indicator comprising at least oneof the intra mode index and a flag value, the flag value indicating themapping of the extension directional intra-prediction mode onto theselected predetermined directional intra-prediction mode, wherein theflag value is used when the intra mode index is associated with ormapped to the extension directional intra-prediction mode, and toassociate the signaling indicator with the extension directionalintra-prediction mode, an intra-prediction unit configured tointra-predict pixel values of pixels of the rectangular video codingblock on the basis of the extension directional intra-prediction modeassociated with the signaling indicator for providing a predictedrectangular video coding block, and an encoding unit configured toencode the rectangular video coding block on the basis of the predictedrectangular video coding block.

The selection of the predetermined directional intra-prediction modefrom the set of directional intra-prediction modes by the mode mappingunit can e.g. be performed such that a complementary direction of theextension directional intra-prediction mode is opposite to apredetermined direction of the selected predetermined directionalintra-prediction mode. For this purpose, a mirroring procedure can beapplied.

In a first implementation form of the encoding apparatus according tothe first aspect as such, each predetermined directionalintra-prediction mode is associated with a predetermined directionwithin a predetermined directional range, wherein the extensiondirectional intra-prediction mode is associated with a complementarydirection within a complementary directional range, and wherein thedirectional range is different from the predetermined directional range.

In this regard, the term “direction” refers to an orientation within thevideo coding block to be used for directional intra-prediction withinthe video coding block. The term “directional range” refers to rangecovering a plurality of said directions.

In a second implementation form of the encoding apparatus according tothe first implementation form of the first aspect, the complementarydirection of the extension directional intra-prediction mode is oppositeto the predetermined direction of the selected predetermined directionalintra-prediction mode.

In a third implementation form of the encoding apparatus according tothe first implementation form or the second implementation form of thefirst aspect, the complementary directional range and the predetermineddirectional range are adjacent.

In a fourth implementation form of the encoding apparatus according tothe first aspect as such or any preceding implementation form of thefirst aspect, the flag value is a binary value.

In a fifth implementation form of the encoding apparatus according tothe first aspect as such or any preceding implementation form of thefirst aspect, the signaling indicator comprises a most-probable-mode(MPM) indicator.

In a sixth implementation form of the encoding apparatus according tothe first aspect as such or any preceding implementation form of thefirst aspect, the rectangular video coding block is a coding unit (CU),a prediction unit (PU), or a transform unit (TU).

According to a second aspect, the invention relates to a decodingapparatus for decoding an encoded rectangular video coding block, thedecoding apparatus being configured to handle a signaling indicatorassociated with an extension directional intra-prediction mode, thesignaling indicator comprising an intra mode index, the intra mode indexbeing associated with a predetermined directional intra-prediction mode.The decoding apparatus comprises an intra-prediction unit configured tointra-predict pixel values of pixels of the encoded rectangular videocoding block on the basis of the extension directional intra-predictionmode associated with the signaling indicator for providing a predictedrectangular video coding block, and a restoration unit configured torestore a rectangular video coding block on the basis of the encodedrectangular video coding block and the predicted rectangular videocoding block.

In a first implementation form of the decoding apparatus according tothe second aspect as such, the signaling indicator further comprises aflag value, the flag value indicating a mapping of the extensiondirectional intra-prediction mode onto the predetermined directionalintra-prediction mode, wherein the flag value is used when the intramode index is associated with or mapped to the extension directionalintra-prediction mode.

According to a third aspect, the invention relates to an encoding methodfor encoding a rectangular video coding block, the encoding method beingconfigured to signal an extension directional intra-prediction modewithin a set of directional intra-prediction modes using a signalingindicator, the set of directional intra-prediction modes comprisingpredetermined directional intra-prediction modes and the extensiondirectional intra-prediction mode. The encoding method comprisesselecting a predetermined directional intra-prediction mode from the setof directional intra-prediction modes in dependence of the extensiondirectional intra-prediction mode, the selected predetermineddirectional intra-prediction mode being associated with an intra modeindex, mapping the extension directional intra-prediction mode onto theselected predetermined directional intra-prediction mode, generating thesignaling indicator comprising at least one of the intra mode index anda flag value, the flag value indicating the mapping of the extensiondirectional intra-prediction mode onto the selected predetermineddirectional intra-prediction mode, wherein the flag value is used whenthe intra mode index is associated with or mapped to the extensiondirectional intra-prediction mode, associating the signaling indicatorwith the extension directional intra-prediction mode, intra-predictingpixel values of pixels of the rectangular video coding block on thebasis of the extension directional intra-prediction mode associated withthe signaling indicator for providing a predicted rectangular videocoding block, and encoding the rectangular video coding block on thebasis of the predicted rectangular video coding block.

The encoding method can be performed by the encoding apparatus. Furtherfeatures of the encoding method directly result from the features or thefunctionality of the encoding apparatus.

According to a fourth aspect, the invention relates to a decoding methodfor decoding an encoded rectangular video coding block, the decodingmethod being configured to handle a signaling indicator associated withan extension directional intra-prediction mode, the signaling indicatorcomprising an intra mode index, the intra mode index being associatedwith a predetermined directional intra-prediction mode. The decodingmethod comprises intra-predicting pixel values of pixels of the encodedrectangular video coding block on the basis of the extension directionalintra-prediction mode associated with the signaling indicator forproviding a predicted rectangular video coding block, and restoring arectangular video coding block on the basis of the encoded rectangularvideo coding block and the predicted rectangular video coding block.

The decoding method can be performed by the decoding apparatus. Furtherfeatures of the decoding method directly result from the features or thefunctionality of the decoding apparatus.

In a first implementation form of the decoding method according to thefourth aspect as such, the signaling indicator further comprises a flagvalue, the flag value indicating a mapping of the extension directionalintra-prediction mode onto the predetermined directionalintra-prediction mode, wherein the flag value is used when the intramode index is associated with or mapped to the extension directionalintra-prediction mode.

According to a fifth aspect, the invention relates to a computer programcomprising program code for performing the method according to the thirdaspect as such or any implementation form of the third aspect or thefourth aspect as such or any implementation form of the fourth aspectwhen executed on a computer.

Embodiments of the invention can be implemented in hardware and/orsoftware.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect tothe following figures, wherein:

FIG. 1 shows a schematic diagram of an encoding apparatus for encoding arectangular video coding block;

FIG. 2 shows a schematic diagram of a decoding apparatus for decoding anencoded rectangular video coding block;

FIG. 3 shows a schematic diagram of an encoding method for encoding arectangular video coding block;

FIG. 4 shows a schematic diagram of a decoding method for decoding anencoded rectangular video coding block;

FIG. 5 shows a schematic diagram of a video coding block illustratingdifferent directional intra-prediction modes;

FIGS. 6(a) and 6(b) illustrate an example of block partitioning and acorresponding tree structure by using quad-tree plus binary-tree (QTBT);

FIGS. 7(a) and 7(b) illustrate implementations of a directionalintra-prediction mechanism in quad-tree (QT) and quad-tree plusbinary-tree (QTBT) frameworks, respectively;

FIGS. 8a and 8b illustrate an extension of a set of directionalintra-prediction modes subject to an aspect ratio of a given rectangularvideo coding block;

FIG. 9 shows a schematic diagram illustrating an extension of a set ofdirectional intra-prediction modes subject to an aspect ratio of a givenrectangular video coding block;

FIG. 10 shows a schematic diagram illustrating a preservation of acardinality of directional intra-prediction modes subject to an aspectratio of a given rectangular video coding block;

FIG. 11 illustrates an example of block partitioning and a correspondingtree structure by using quad-tree plus binary-tree (QTBT), wherein thenumber of available reference samples along a longer side is less thanits double length in a rectangular video coding block;

FIG. 12 illustrates enabling or disabling a set of directionalintra-prediction modes subject to an availability of reference samplesof a given rectangular video coding block;

FIG. 13 illustrates a first step of a signaling mechanism for extensionof directional intra-prediction modes;

FIG. 14 illustrates a second step of a signaling mechanism for extensionof directional intra-prediction modes;

FIG. 15 illustrates a decoding process for a directional intra modeindex by applying a signaling mechanism;

FIG. 16 shows a schematic diagram illustrating an implementation of asignaling mechanism applied in an encoding apparatus;

FIG. 17 shows a schematic diagram illustrating an implementation of asignaling mechanism applied in a decoding apparatus;

FIGS. 18(a) and 18(b) show schematic diagrams illustratingimplementations of a signaling mechanism applied to the EnhancedIntra-Prediction (EIP) mechanism; and

FIG. 19 shows a schematic diagram of an encoding apparatus comprising amode mapping unit, a signaling unit, and an intra-prediction unit.

In the various figures, identical reference signs will be used foridentical or at least functionally equivalent features.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings, which form part of the disclosure, and in which are shown, byway of illustration, specific aspects in which embodiments of thepresent invention may be placed. It is understood that other aspects maybe utilized and structural or logical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description, therefore, is not to be taken in a limiting sense,as the scope of the present invention is defined be the appended claims.

For instance, it is understood that a disclosure in connection with adescribed method may also hold true for a corresponding device or systemconfigured to perform the method and vice versa. For example, if aspecific method step is described, a corresponding device may include aunit to perform the described method step, even if such unit is notexplicitly described or illustrated in the figures. Further, it isunderstood that the features of the various exemplary aspects describedherein may be combined with each other, unless specifically notedotherwise.

FIG. 1 shows a schematic diagram of an encoding apparatus 100 forencoding a rectangular video coding block. The encoding apparatus 100 isconfigured to signal an extension directional intra-prediction modewithin a set of directional intra-prediction modes using a signalingindicator, the set of directional intra-prediction modes comprisingpredetermined directional intra-prediction modes and the extensiondirectional intra-prediction mode. The encoding apparatus 100 comprisesa mode mapping unit 101 configured to select a predetermined directionalintra-prediction mode from the set of directional intra-prediction modesin dependence of the extension directional intra-prediction mode, theselected predetermined directional intra-prediction mode beingassociated with an intra mode index, and to map the extensiondirectional intra-prediction mode onto the selected predetermineddirectional intra-prediction mode, a signaling unit 103 configured togenerate the signaling indicator comprising at least one of the intramode index and a flag value, the flag value indicating the mapping ofthe extension directional intra-prediction mode onto the selectedpredetermined directional intra-prediction mode, wherein the flag valueis used when the intra mode index is associated with or mapped to theextension directional intra-prediction mode, and to associate thesignaling indicator with the extension directional intra-predictionmode, an intra-prediction unit 105 configured to intra-predict pixelvalues of pixels of the rectangular video coding block on the basis ofthe extension directional intra-prediction mode associated with thesignaling indicator for providing a predicted rectangular video codingblock, and an encoding unit 107 configured to encode the rectangularvideo coding block on the basis of the predicted rectangular videocoding block.

FIG. 2 shows a schematic diagram of a decoding apparatus 200 fordecoding an encoded rectangular video coding block. The decodingapparatus 200 is configured to handle a signaling indicator associatedwith an extension directional intra-prediction mode, the signalingindicator comprising an intra mode index, the intra mode index beingassociated with a predetermined directional intra-prediction mode. Thedecoding apparatus 200 comprises an intra-prediction unit 201 configuredto intra-predict pixel values of pixels of the encoded rectangular videocoding block on the basis of the extension directional intra-predictionmode associated with the signaling indicator for providing a predictedrectangular video coding block, and a restoration unit 203 configured torestore a rectangular video coding block on the basis of the encodedrectangular video coding block and the predicted rectangular videocoding block. The signaling indicator can further comprise a flag value,the flag value indicating a mapping of the extension directionalintra-prediction mode onto the predetermined directionalintra-prediction mode.

FIG. 3 shows a schematic diagram of an encoding method 300 for encodinga rectangular video coding block. The encoding method 300 beingconfigured to signal an extension directional intra-prediction modewithin a set of directional intra-prediction modes using a signalingindicator, the set of directional intra-prediction modes comprisingpredetermined directional intra-prediction modes and the extensiondirectional intra-prediction mode. The encoding method 300 comprisesselecting 301 a predetermined directional intra-prediction mode from theset of directional intra-prediction modes in dependence of the extensiondirectional intra-prediction mode, the selected predetermineddirectional intra-prediction mode being associated with an intra modeindex, mapping 303 the extension directional intra-prediction mode ontothe selected predetermined directional intra-prediction mode, generating305 the signaling indicator comprising at least one of the intra modeindex and a flag value, the flag value indicating the mapping of theextension directional intra-prediction mode onto the selectedpredetermined directional intra-prediction mode, wherein the flag valueis used when the intra mode index is associated with or mapped to theextension directional intra-prediction mode, associating 307 thesignaling indicator with the extension directional intra-predictionmode, intra-predicting 309 pixel values of pixels of the rectangularvideo coding block on the basis of the extension directionalintra-prediction mode associated with the signaling indicator forproviding a predicted rectangular video coding block, and encoding 311the rectangular video coding block on the basis of the predictedrectangular video coding block.

FIG. 4 shows a schematic diagram of a decoding method 400 for decodingan encoded rectangular video coding block. The decoding method 400 beingconfigured to handle a signaling indicator associated with an extensiondirectional intra-prediction mode, the signaling indicator comprising anintra mode index, the intra mode index being associated with apredetermined directional intra-prediction mode. The decoding method 400comprises intra-predicting 401 pixel values of pixels of the encodedrectangular video coding block on the basis of the extension directionalintra-prediction mode associated with the signaling indicator forproviding a predicted rectangular video coding block, and restoring 403a rectangular video coding block on the basis of the encoded rectangularvideo coding block and the predicted rectangular video coding block. Thesignaling indicator can further comprise a flag value, the flag valueindicating a mapping of the extension directional intra-prediction modeonto the predetermined directional intra-prediction mode.

FIG. 5 shows a schematic diagram of a video coding block illustratingdifferent directional intra-prediction modes. The intra prediction modesas shown in FIG. 5 include a planar mode (the intra-prediction modeindex is 0), DC mode (the intra-prediction mode index is 1), and 33directional modes (the intra-prediction mode index ranges from 2 to 34,indicated by the solid lines). The set of directional intra-predictionmodes was extended up to 65 modes (almost doubled) by decreasing a stepangle between directional intra-prediction modes by a factor of 2. Thedotted lines in FIG. 5 denote the angular modes, which are introduced inthe JEM software.

FIGS. 6a and 6b illustrate an example of block partitioning and acorresponding tree structure by using quad-tree plus binary-tree (QTBT),wherein solid lines denote quad-tree partitioning and dashed linesdenote binary-tree partitioning. In each partitioning node of thebinary-tree, the partitioning type is indicated by 0 (horizontalpartitioning) or 1 (vertical partitioning).

FIGS. 7a and 7b illustrate implementations of a directionalintra-prediction mechanism in quad-tree (QT) and quad-tree plusbinary-tree (QTBT) frameworks, respectively. Here, the same number ofreference samples are used along both shorter and longer sides ofrectangular blocks. Therefore, the number of directionalintra-prediction modes depends on neither aspect ratio of blocks noractual availability of reference samples in the current implementationof the QTBT framework.

FIGS. 8a and 8b illustrate an extension of a set of directionalintra-prediction modes subject to an aspect ratio of a given rectangularvideo coding block. As shown in FIG. 8a , an aspect ratio of a squarevideo coding block is 1:1 and a set of conventional directionalintra-prediction modes is used for predicting values of a video codingblock being reconstructed. On the other hand, a rectangular video codingblock comprises shorter and longer sides, and such asymmetry can be usedto improve the current directional intra-prediction mechanism byincreasing its prediction accuracy. As illustrated in FIG. 8b , thenumber of available directional intra-prediction modes can be increasedalong a long side.

FIG. 9 shows a schematic diagram illustrating an extension of a set ofdirectional intra-prediction modes subject to an aspect ratio of a givenrectangular video coding block. The corresponding processing steps maybe implemented by the intra-prediction apparatus 100 and/or theintra-prediction method 400. In FIG. 9, square pixels representreference samples for intra-prediction, wherein the order ofprobabilities that the reference samples are available is: referencepixel with dots>reference pixel with stripes>reference pixel withdiagonal stripes.

The number of the newly introduced directional intra-prediction modesmay depend on the aspect ratio of the rectangular video coding block.The angle that encompasses these new modes is defined by the followingformula:

$\alpha = {\frac{\pi}{4} - {\arctan \left( \frac{L_{shorter}}{L_{longer}} \right)}}$

wherein L_(shorter) and L_(longer) are the lengths of the shorter andlonger sides of the rectangular video coding block, respectively. Asillustrated in FIG. 9, L_(shorter)=width and L_(longer)=height for avertical orientation of the rectangular video coding block. The actualnumber of these modes may depend on the angle between neighbordirectional modes and the angle α defined by the above formula.

In the up-to-date version of the JEM software (version JEM-4.0), theaverage angle step between neighbor directional modes defined by anintra-prediction interpolation filter does not depend on the block sizeand equals:

$s = \frac{\pi}{64}$

Thus, in the case of uniformly spaced directional intra-predictionmodes, the number N of the newly introduced modes equals:

$N = {\left\lfloor \frac{\alpha}{s} \right\rbrack = {16 - \left\lfloor {\frac{64}{\pi}{\arctan \left( \frac{L_{shorter}}{L_{longer}} \right)}} \right\rfloor}}$

wherein └⋅┘ is a floor operation.

In the embodiment shown in FIG. 9, the number of reference samples isextended along the longer side, and it is not reduced for the shorterside. Therefore, the amount of intra-prediction modes that are availablealong the longer side (the angle that encompasses these modes is markedby a solid line) is increased, but the number of intra-prediction modesthat are available along the shorter side (the angle that encompassesthese modes is marked by a dashed line) is not decreased. Hence, thecardinality of the intra-prediction mode set is only increased while theaspect ratio

$R_{asp} = \frac{L_{shorter}}{L_{longer}}$

is decreasing. On the other hand, another approach to preserve theoriginal number of directional intra-prediction modes is also possibleaccording to another embodiment.

FIG. 10 shows a schematic diagram illustrating a preservation of acardinality of directional intra-prediction modes subject to an aspectratio of a given rectangular video coding block. As shown in FIG. 10,the amount of the directional intra-prediction modes added along thelonger side (the angle that encompasses these modes is marked by a solidline) may be equal to the amount of the directional intra-predictionmodes removed along the shorter side (the angle that encompasses thesemodes is marked by a dashed lines). Thus, the cardinality of theintra-prediction mode set remains the same as for square blocks.

According to an embodiment, whether to extend a set of availableintra-prediction modes or not can also depend on the availability ofreference samples because they are needed to generate anintra-predictor.

FIG. 11 illustrates an example of block partitioning and a correspondingtree structure by using quad-tree plus binary-tree (QTBT), wherein thenumber of available reference samples along a longer side is less thanits double length in a rectangular video coding block. As shown in FIG.11, the quad-tree plus binary-tree (QTBT) partitioning frameworkproduces a partitioning, wherein the actual number of availablereference samples along a longer side is less than its double length asassumed in the above examples in FIGS. 9 and 10. Therefore, the approachfor increasing the number of the directional intra-prediction modes inthe above examples may need to be adjusted according to an availabilityof reference samples for the case of FIG. 11.

FIG. 12 illustrates enabling or disabling a set of directionalintra-prediction modes subject to an availability of reference samplesof a given rectangular video coding block within the quad-tree plusbinary-tree (QTBT) partitioning framework, wherein a grey rectangle arearepresents a currently processed video coding block, square pixels withdiagonal stripes indicate available reference samples, and square pixelswith dots indicate unavailable reference samples. Disabling can e.g. beachieved by removing a respective directional intra-prediction mode fromthe set.

A fractional non-prediction area P of a rectangular video coding blockgenerated using interpolated reference samples may be calculated asfollows:

$\begin{matrix}{P_{area} = {\frac{S_{uncov}}{S_{block}} = \frac{S_{uncov}}{L_{shorter} \cdot L_{longer}}}} \\{= {\frac{{L_{shorter}^{2} \cdot \tan}\; \gamma}{2 \cdot L_{shorter} \cdot L_{longer}} =}} \\{= {\frac{{L_{shorter} \cdot \tan}\; \gamma}{2 \cdot L_{longer}} = {\frac{L_{shorter}}{L_{longer}} \cdot \frac{\tan \; \gamma}{2}}}} \\{= {R_{asp} \cdot \frac{\tan \; \gamma}{2}}}\end{matrix}$

wherein L_(longer) and L_(shorter) are the lengths of the longer andshorted sides of a rectangular video coding block, respectively, γ isthe angle of a given directional intra-prediction mode belonging to theextended set, S_(block)=L_(shorter)·L_(longer) is the area of arectangular video coding block to be predicted,

$S_{uncov} = \frac{{L_{shorter}^{2} \cdot \tan}\; \gamma}{2}$

is the non-prediction area, i.e. the area of the video coding block thatmay not be predicted using non-interpolated reference samples, as markedby stripes.

Therefore, the closer an intra-prediction direction is located to thediagonal marked by a dashed line, the larger part of an area thatremains may not be predicted using non-interpolated reference samples.In an example, the set of directional intra-prediction modes is notextended if the length L_(RSlonger) of non-interpolated referencesamples along the longer side is less than the double length of thelonger side:

L_(RSlonger)<2L_(longer).

If a set of directional intra-prediction modes is extended, it isdesirable to signal the newly extended modes, which may not beaccomplished using existing conventional mechanisms. For this purpose, a2-step signaling mechanism for the extension of directionalintra-prediction modes is proposed and explained in FIGS. 13 and 14.

FIG. 13 illustrates a first step of a signaling mechanism for extensionof directional intra-prediction modes, wherein a set of extended modesis mapped to a conventional set of intra-prediction modes using amirroring procedure.

FIG. 14 illustrates a second step of a signaling mechanism for extensionof directional intra-prediction modes, wherein in a one-bit flag is usedto distinguish between conventional and extended directional modes. Theflag is assigned a value “0” for a conventional mode and “1” for anextended mode. Furthermore, the flag in the signaling mechanism is usedonly for those directional modes that are reflections of extended ones.

FIG. 15 illustrates a decoding process for a directional intra modeindex by applying a signaling mechanism. As shown in FIG. 15, theextended modes of the directional intra-prediction are flagged with “1”,the conventional modes having a mapped mode are flagged with “0”, andthe other modes have no additional signaling value.

FIG. 16 shows a schematic diagram illustrating an implementation of asignaling mechanism applied in an encoding apparatus. In a firstprocessing step 1601 the index of the intra-prediction mode I_(IPM) isparsed from the bitstream. Thereafter, in processing step 1603 adecision is taken depending on whether the decoded intra-prediction modeis a directional intra prediction mode. In the case the signaling schemeis applied in the context of HEVC video coding, the intra-predictionmode is directional when I_(IPM) is greater than 1. If theintra-prediction mode is directional, in processing step 1605 a decisionis taken depending on whether the decoded intra-prediction mode isextended. The decoded intra-prediction mode is extended when I_(IPM) isgreater than Q[π/2+arctan(Width/Height)] and smaller than VDIAG_IDX,wherein Width and Height are the lengths of short and long sides of arectangular video coding block being decoded, and VDIAG_IDX is equal to66 according to embodiments of the invention. Then, the flag“ext_dir_mode_flag” is assigned to a value of 0 for the conventionalmodes which can have mapped extended code (see processing steps 1607,1609). A rate-distortion cost (RD-cost) is estimated for theconventional modes in processing step 1611. The flag “ext_dir_mode_flag”is assigned to a value of 1 for the extended modes (see processing steps1613, 1615). A rate-distortion cost (RD-cost) for the conventional modesis estimated in processing step 1617. The flag “ext_dir_mode_flag” isdetermined by finding the lowest rate-distortion cost (RD-cost) betweenthe conventional modes and extended modes in processing step 1619.

FIG. 17 shows a schematic diagram illustrating an implementation of asignaling mechanism applied in a decoding apparatus. In a firstprocessing step 1701 the index of the intra-prediction mode I_(IPM) isparsed from the bitstream. Thereafter, in processing step 1703 adecision is taken depending on whether the decoded intra prediction modeis a directional intra prediction mode. In the case the signaling schemeis applied in the context of HEVC video coding, the intra predictionmode is directional when I_(IPM) is greater than 1. If theintra-prediction mode is directional, in processing step 1705 a decisionis taken depending on whether the decoded intra-prediction mode isextended. The decoded intra-prediction mode is extended when I_(IPM) isgreater than Q[π/2+arctan(Width/Height)] and smaller than VDIAG_IDX,wherein Width and Height are the lengths of short and long sides of arectangular block being decoded, and VDIAG_IDX is equal to 66 accordingto embodiments of the invention. For extended directionalintra-prediction modes the value of the flag “ext_dir_mode_flag” isparsed from the bitstream in processing step 1707. According toembodiments of the invention this flag is introduced into the bitstreamto code whether to apply the proposed mechanism to the prediction unit.In processing step 1709, a decision is taken to use either the extendedprediction scheme if ext_dir_mode_flag is equal to 1 (processing step1711 a) or the conventional prediction if ext_dir_mode_flag is not equalto 1 (processing step 1711 b), as provided by embodiments of theinvention, for obtaining the predicted signal. The decision inprocessing step 1709 is taken on the basis of the value of the flag“ext_dir_mode_flag”, which has been determined in processing step 1707.

The signaling mechanism is applicable to a wider spectrum of casesaccording to embodiments of the invention. For examples, it can be usedto reduce a signaling overhead caused by an extended set of directionalintra-prediction modes used in Enhanced Intra-Prediction (EIP) techniqueproposed by Google for its VPX codec family. This EIP technique isneeded to improve the compression efficiency of intra-predicted blockswithin inter-predicted pictures. EIP is a two-pass mechanism forincreasing the number of available prediction directions, wherein blockswith good inter-prediction modes are initially encoded, and then intrablocks with access to more boundaries are filled in.

FIGS. 18a and 18b show schematic diagrams illustrating implementationsof a signaling mechanism applied to the Enhanced Intra-Prediction (EIP)mechanism. In the cases shown in FIGS. 18a and 18 b, 4 (2π) and 3 (3π/2)sides of a video coding block are available for directionalintra-prediction, respectively. Solid lines denote directions from amain angle and dashed lines denote directions from a complimentaryangle. In both cases, the set of available intra-prediction modes ismore than for a conventional case.

As described above, the same 2-step signaling mechanism can be conductedto signal what angle the selected directional intra-prediction modebelongs to by using a one-bit flag. Firstly, a directional mode can bemapped onto the main angle if the directional mode is selected from thecomplementary angle. Secondly, the one-bit flag can be set to “ON” ifthe direction is selected from the complementary angle; otherwise, theflag can be set to “OFF”.

FIG. 19 shows a schematic diagram of an encoding apparatus 100comprising a mode mapping unit 101, a signaling unit 103, and anintra-prediction unit 105. A decoding apparatus 200 can be implementedanalogously.

While a particular feature or aspect of the disclosure may have beendisclosed with respect to only one of several implementations orembodiments, such a feature or aspect may be combined with one or morefurther features or aspects of the other implementations or embodimentsas may be desired or advantageous for any given or particularapplication. Furthermore, to the extent that the terms “include”,“have”, “with”, or other variants thereof are used in either thedetailed description or the claims, such terms are intended to beinclusive in a manner similar to the term “comprise”. Also, the terms“exemplary”, “for example” and “e.g.” are merely meant as an example,rather than the best or optimal. The terms “coupled” and “connected”,along with derivatives thereof may have been used. It should beunderstood that these terms may have been used to indicate that twoelements cooperate or interact with each other regardless whether theyare in direct physical or electrical contact, or they are not in directcontact with each other.

Although specific aspects have been illustrated and described herein, itwill be appreciated that a variety of alternate and/or equivalentimplementations may be substituted for the specific aspects shown anddescribed without departing from the scope of the present disclosure.This application is intended to cover any adaptations or variations ofthe specific aspects discussed herein.

Although the elements in the following claims are recited in aparticular sequence with corresponding labeling, unless the claimrecitations otherwise imply a particular sequence for implementing someor all of those elements, those elements are not necessarily intended tobe limited to being implemented in that particular sequence.

Many alternatives, modifications, and variations will be apparent tothose skilled in the art in light of the above teachings. Of course,those skilled in the art readily recognize that there are numerousapplications of embodiments of the invention beyond those describedherein. While embodiments of the present invention have been describedwith reference to one or more particular embodiments, those skilled inthe art recognize that many changes may be made thereto withoutdeparting from the scope of the present invention. It is therefore to beunderstood that within the scope of the appended claims and theirequivalents, the invention may be practiced otherwise than asspecifically described herein.

What is claimed is:
 1. An encoding apparatus for encoding a rectangularvideo coding block, the encoding apparatus being configured to signal anextension directional intra-prediction mode within a set of directionalintra-prediction modes using a signaling indicator, the set ofdirectional intra-prediction modes comprising predetermined directionalintra-prediction modes and the extension directional intra-predictionmode, the encoding apparatus comprising: a mode mapping unit configuredto select a predetermined directional intra-prediction mode from the setof directional intra-prediction modes in dependence of the extensiondirectional intra-prediction mode, the selected predetermineddirectional intra-prediction mode being associated with an intra modeindex, and to map the extension directional intra-prediction mode ontothe selected predetermined directional intra-prediction mode; asignaling unit configured to generate the signaling indicator comprisingat least one of the intra mode index and a flag value, the flag valueindicating the mapping of the extension directional intra-predictionmode onto the selected predetermined directional intra-prediction mode,wherein the flag value is used when the intra mode index is associatedwith the extension directional intra-prediction mode, and to associatethe signaling indicator with the extension directional intra-predictionmode; an intra-prediction unit configured to intra-predict pixel valuesof pixels of the rectangular video coding block on the basis of theextension directional intra-prediction mode associated with thesignaling indicator for providing a predicted rectangular video codingblock; and an encoding unit configured to encode the rectangular videocoding block on the basis of the predicted rectangular video codingblock.
 2. The encoding apparatus of claim 1, wherein each predetermineddirectional intra-prediction mode is associated with a predetermineddirection within a predetermined directional range, wherein theextension directional intra-prediction mode is associated with acomplementary direction within a complementary directional range, andwherein the directional range is different from the predetermineddirectional range.
 3. The encoding apparatus of claim 2, wherein thecomplementary direction of the extension directional intra-predictionmode is opposite to the predetermined direction of the selectedpredetermined directional intra-prediction mode.
 4. The encodingapparatus of claim 2, wherein the complementary directional range andthe predetermined directional range are adjacent.
 5. The encodingapparatus of claim 1, wherein the flag value is a binary value.
 6. Theencoding apparatus of claim 1, wherein the signaling indicator comprisesa most-probable-mode indicator.
 7. The encoding apparatus of claim 1,wherein the rectangular video coding block is a coding unit, aprediction unit, or a transform unit.
 8. A decoding apparatus fordecoding an encoded rectangular video coding block, the decodingapparatus being configured to handle a signaling indicator associatedwith an extension directional intra-prediction mode, the signalingindicator comprising an intra mode index, the intra mode index beingassociated with a predetermined directional intra-prediction mode, thedecoding apparatus comprising: an intra-prediction unit configured tointra-predict pixel values of pixels of the encoded rectangular videocoding block on the basis of the extension directional intra-predictionmode associated with the signaling indicator for providing a predictedrectangular video coding block; and a restoration unit configured torestore a rectangular video coding block on the basis of the encodedrectangular video coding block and the predicted rectangular videocoding block.
 9. The decoding apparatus of claim 8, wherein thesignaling indicator further comprises a flag value, the flag valueindicating a mapping of the extension directional intra-prediction modeonto the predetermined directional intra-prediction mode, wherein theflag value is used when the intra mode index is associated with theextension directional intra-prediction mode.
 10. An encoding method forencoding a rectangular video coding block, the encoding method beingconfigured to signal an extension directional intra-prediction modewithin a set of directional intra-prediction modes using a signalingindicator, the set of directional intra-prediction modes comprisingpredetermined directional intra-prediction modes and the extensiondirectional intra-prediction mode, the encoding method comprising:selecting a predetermined directional intra-prediction mode from the setof directional intra-prediction modes in dependence of the extensiondirectional intra-prediction mode, the selected predetermineddirectional intra-prediction mode being associated with an intra modeindex; mapping the extension directional intra-prediction mode onto theselected predetermined directional intra-prediction mode; generating thesignaling indicator comprising at least one of the intra mode index anda flag value, the flag value indicating the mapping of the extensiondirectional intra-prediction mode onto the selected predetermineddirectional intra-prediction mode, wherein the flag value is used whenthe intra mode index is associated with the extension directionalintra-prediction mode; associating the signaling indicator with theextension directional intra-prediction mode; intra-predicting pixelvalues of pixels of the rectangular video coding block on the basis ofthe extension directional intra-prediction mode associated with thesignaling indicator for providing a predicted rectangular video codingblock; and encoding the rectangular video coding block on the basis ofthe predicted rectangular video coding block.
 11. A decoding method fordecoding an encoded rectangular video coding block, the decoding methodbeing configured to handle a signaling indicator associated with anextension directional intra-prediction mode, the signaling indicatorcomprising an intra mode index, the intra mode index being associatedwith a predetermined directional intra-prediction mode, the decodingmethod comprising: intra-predicting pixel values of pixels of theencoded rectangular video coding block on the basis of the extensiondirectional intra-prediction mode associated with the signalingindicator for providing a predicted rectangular video coding block; andrestoring a rectangular video coding block on the basis of the encodedrectangular video coding block and the predicted rectangular videocoding block.
 12. The decoding method of claim 11, wherein the signalingindicator further comprises a flag value, the flag value indicating amapping of the extension directional intra-prediction mode onto thepredetermined directional intra-prediction mode, wherein the flag valueis used when the intra mode index is associated with the extensiondirectional intra-prediction mode.
 13. A non-transitory computerreadable medium comprising program code for performing the method ofclaim 10 when executed on a computer.
 14. A non-transitory computerreadable medium comprising program code for performing the method ofclaim 11 when executed on a computer.